This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a rotating polygon to a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to dynamic switching between start of scan (SOS) sensor and Hall sensors as feedback signals for polygon speed control.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Laser printers may operate in either a single pass or multiple pass system.
In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scanline formed by the modulated laser beam generated by a ROS system. Each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (slow scan registration) and in the direction perpendicular to the process direction (referred to as fast scan or lateral registration).
In systems requiring such precision, speed control of the rotating polygon is essential. In pending application D/94326, U.S. Ser. No. 08/510,998 filed Aug. 3, 1995, assigned to the same assignee as the present invention, there is disclosed a control that provides a start of scan (SOS) signal for each of the facets of a rotating polygon. The control determines the facet related to the first scanline of a first image exposure frame on a photoconductive member, and initiates the first scanline of each succeeding superimposed image exposure frame on the photoconductive member in relation to the facet related to the first scanline of the first image exposure frame. A time period measurement between a given facet occurrence to the same given facet repeat occurrence, relative to subsequent full revolutions of the polygon, provides an `error free` electronic representation of the speed of the polygon.
A difficulty, however, in the prior art is that the start of scan (SOS) signals are only provided by sensing a portion of the projected laser beam. When the laser is off, there are no light beam signals to be sensed to be able to control polygon speed. Some level of relatively inexpensive speed control is needed when the laser beam is off is necessary to be able to uniformly commence SOS control after the laser is on.
Thus, it would be desirable to provide a relatively inexpensive polygon scanning system that sufficiently controls polygon speed even if the laser scanning system is in an off state. It is an object of the present invention, therefore, to provide two levels of polygon speed control, a less precise level of speed control, such as using Hall sensors, with the laser in an off state and a very precise speed control, SOS signals, when the laser is in an on state. It is another object of the present invention to provide a first level of polygon speed control when the laser device is off to enable an easy transition to SOS signal speed control when the laser device is on. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.